Purification of chromium plating solutions using electrodialysis is well-known in the art (see U.S. Pat. Nos. 3,481,851; 3,909,381; and 4,006,067, the disclosures of which are hereby incorporated by reference). Electrodialysis is the transport of ions through an ion permeable membrane as a result of an electrical driving force, and the process is commonly carried out in an electrodialysis cell having an anolyte compartment and a catholyte compartment separated by a permselective membrane. The permselective membranes are not unlike ion exchange resins in sheet or membrane form. They comprise a matrix of a chemically inert resin throughout the polymer lattice of which are distributed chemically bound anionic or cationic moieties having fixed negative and positive charges. Anion permeable membranes have positive (cationic) fixed charges distributed throughout the polymer lattice and, as the name implies, are permeable to negatively charged ions and are relatively impermeable to positively charged ions. Unfortunately, there are no known anion permeable membranes that are 100% impermeable to cations, and there are no known cation permeable membranes that are 100% impermeable to anions. As a result, there is always in every electrodialysis process some small degree of reverse migration of cations through the anion permeable membrane and/or of anions through the cation permeable membrane.
U.S. Pat. No. 3,481,851 teaches that the dissolved metallic contaminants can be removed from the aqueous chromium plating solution by electrodialysis. An electric current is passed between the anode and the cathode of the cell through the aqueous solutions contained in the anolyte and catholyte compartments of the cell. The electric current causes the contaminant metal cations (for example, iron and copper ions) present in the chromic acid solution to migrate from the anolyte compartment through the cation permeable membrane into the catholyte compartment, reverse migration of anions (for example, chloride ions) being prevented, in theory at least, by the cation permeable membrane. This process effectively reduces the concentration of contaminant metal cations in the chromic acid solution to acceptable levels. In addition, the electrolytic oxidizing conditions prevailing in the anolyte quickly oxidizes the trivalent chromium present therein to the hexavalent state, thereby reducing the ratio of trivalent to hexavalent chromium to an acceptable level. However, the cation permeable membrane also permits the reverse migration of a small amount of mineral acid anions from the catholyte to the anolyte compartment and as a consequence there is a fairly rapid build-up of these anions in the chromium plating solution. The build-up of mineral acid anions in the anolyte quickly renders the chromic acid solution unsuitable for chromium plating. Therefore, while this process will effectively remove harmful metal cations (for example, iron and copper ions) from the chromium plat-solution, it also results in the rapid build-up of equally harmful mineral acid anions (for example, chloride ions) in the plating solution. As a result, this process does not provide a satisfactory solution to the problem of rejuvenating chromium plating solutions by the removal of contaminant metal cations therefrom.
U.S. Pat. No. 3,909,381 teaches that the metallic contaminants can be removed from the chromium plating systems in an electrodialysis cell wherein the catholyte comprises an aqueous solution of at least one ionizable organic compound and wherein the anions of the ionizable organic compounds in the catholyte are oxidized to gaseous oxidation products and water when reacted with the chromic acid-containing anolyte thereby reducing the anion contaminants in the anolyte. However, the oxidation of the organic compound results in the reduction of hexavalent chromium to a lower valent chromium which has an adverse effect on the plating performance of the chromium solution. In addition the electrical conductivity of aqueous solutions of organic compound salts is low and, in turn, limits the capacity and electrical efficiency of the electrodialysis cell,